This section provides background information related to the present disclosure which is not necessarily prior art. This section also provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
Scanning stages are used for precise positioning in a variety of advanced manufacturing processes, such as laser patterning, 3-D printing, and pick-and-place type applications for hard drive manufacturing. In particular, they are used for precise positioning at various stages of silicon wafer processing, such as optical lithography and inspection.
In response to increased throughput demands, wafer scanning stages must deliver high accelerations/decelerations (acc/dec) at motion reversals. The resulting high inertial forces that are borne by the linear motor actuators cause Joule heating proportional to the square of the motor current, leading to increased thermal errors. Various methods such as forced cooling, thermal error compensation, light-weighting and optimal control of the motor drives can be used to mitigate thermal errors. Unfortunately, forced cooling requires cooling circuits and external heat exchangers, which add to design complexities and raise costs. Effective thermal error compensation requires reliable thermal models and temperature sensor networks. Light-weighting could reduce structural stiffness and introduce unwanted vibrations. Control techniques can only offer incremental benefits for a given motor design.
In addition to generating excessive heat, the high inertial forces in scanning stages cause residual vibration of the machine frame, which adversely affects positioning speed and precision. Various methods such as tuned mass dampers, input shaping and counter motion devices can be employed to mitigate residual vibration. Tuned mass dampers and input shapers lose effectiveness when operating conditions change. Counter motion devices are bulky, expensive and energy intensive.
A passive assist device (PAD) is a spring mounted in series or parallel with an active element (e.g., motor). A passive assist device consisting of a torsional spring in parallel with a rotary motor has been illustrated to significantly reduce motor currents and power, when properly tuned for a family of motion trajectories. However, the passive assist device could increase motor currents/heat for operating conditions other than the ones for which it was tuned, making it limited in versatility.
According to the principles of the present teachings, a passive assist device is provided that uses magnetic repulsion to simultaneously reduce vibration and heat during motion reversals in wafer scanning. In some embodiments, a pair of repelling permanent magnets is used to store and release the stage's kinetic energy during deceleration and acceleration, respectively, to alleviate motor force requirements thereby reducing heat. In some embodiments, residual vibrations are lessened by channeling the assistive forces provided by the magnets to the ground, instead of to the vibration-sensitive machine base. The magnets can be automatically positioned to provide optimal assist for a given scan trajectory, thus enhancing the versatility of the passive assist device. The following discussion describes the magnet-based passive assist device in greater detail, including the design, sizing and control of a prototype magnet assisted stage. Experimental results obtained from an exemplary stage are presented and discussed.
Furthermore, in some embodiments, a magnet assisted stage system is provided for scanning applications having a scanning table being moveable from a first position to a second position, a scanning actuator operably associated with the scanning table to move the scanning table along a scanning direction from the first position to the second position, and an actively variable magnetic spring system being operably augmented to the scanning table to exert a magnetic repulsion force upon the scanning table in the scanning direction.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.